Field of the Invention
The present invention relates to drill bits for boring subterraneous formations, and specifically to fixed cutter rotary drag bits.
State of the Art
Fixed cutter rotary drag bits have been employed for boring subterraneous formations for many decades, and are particularly suited for drilling oil and gas wells, where they are widely employed. Over the past ten to fifteen years, synthetic diamond cutting elements, generally referred to as polycrystalline diamond compacts, or PDC's, have become the most widely employed cuter type. Such cutting elements may be square, triangular, or other polyhedral shape, but the predominant design is a circular PDC having a planar cutting face backed by a disc or cylinder of metal, such as tungsten carbide. Such cutting elements are secured to the crown or face of the drill bits either by brazing into pockets on the bit face, or by securing the element to a stud which is then inserted in a socket in the bit face. Recently, circular PDC elements which are thermally stable to relatively high temperatures have become available, and such elements may be secured into the face of a matrix-type bit at the time the bit is furnaced.
It is possible to calculate with some accuracy the force generated by a cutter engaging a formation and to break the force into triaxial components, two of which, tangential and radial forces (the third being along the axis of the bit) act into or away from the center of the bit. When summed with those like forces of the other cutters on the face of the bit, the magnitude and direction of the total side forces in a plane perpendicular to the bit axis may be determined. Such calculations generally take into consideration the cutting element size, radial placement, back (or forward) rake, as well as anticipated rotational speed of the bit, weight on the bit and formation type. It is thereby possible to ascertain with some certainty whether or not the sum of the side forces will tend to cause bit imbalance, which sometimes will cause a bit to "wobble" in the wellbore, cutting an oversize borehole, or to "walk" in a particular direction, so that the borehole gradually deviates from a linear path. A fairly detailed explanation of the manner in which such forces may be calculated and summed to indicate any force imbalance is provided by U.S. Pat. No. 4,815,342, assigned to Amoco Corporation.
In former times, where seismic, logging and survey techniques were less refined, borehole deviation was often unrecognized, or was not perceived as a problem. However, in the current drilling environment, wherein seismic logging and survey techniques have been developed to such a degree that potential producing formations may be identified with an enhanced degree of certainty, the industry has recognized that drill bits with greater drilling accuracy are highly desirable.
Accuracy is further desirable due to the current tendency to drill infill wells in existing fields, and to drill a large number of wells, in some cases as many as one hundred, from a single offshore platform. In both situations, intersection of an existing borehole by a second borehole being drilled will result in a blowout, with catastrophic property damage, injury to personnel if not loss of life, and severe environmental damage.
Finally, current industry practice of deploying so-called steerable drilling systems including a downhole motor, whereby the operator may drill both linear and non-linear boreholes using the same bottom hole assembly, places a premium on drill bits which hold to the course directed by the bottom hole assembly.
In addition to the aforementioned problems of bit wobble and bit walk, it has also been observed that a bit having imbalanced cutter side forces may rotate or "whirl" in the borehole about a center point offset from the geometric center of the bit in such a manner that the bit tends to whirl backwards about the borehole. The whirl phenomenon has been observed to be aggravated by the presence of gage cutters or trimmers at certain locations on the gage of the bit, which also generate frictional forces during drilling. Whirl is a dynamic and self-sustaining phenomenon, and in many instances is highly destructive to the drill bit cutters.
Several different approaches have been taken to producing a bit that will drill without the aforementioned performance deficiencies, both of which are exemplified by U.S. patents issued to Amoco Corporation. In one case, as disclosed in previously-referenced U.S. Pat. No. 4,815,342, a bit is produced, the cutting element locations and other parameters are measured, the resultant forces calculated, and one or more cutting elements added to bring the bit into a balanced state with respect to the lateral or side forces generated by the cutting elements. Stated another way, the method seeks to produce a zero force vector in the plane perpendicular to the axis of the bit. A second approach, as disclosed in U.S. Pat. No. 5,010,789, is to place and orient the cutting elements so that the lateral or side force vector is intentionally directed to one side of the bit, which includes a bearing surface in substantially constant contact with the wall of the borehole. This latter methodology evolved from the industrial technique of "gun drilling," used to drill perfectly straight bores in heavy guns such as are employed on naval vessels. Bits employing a directed side force vector in the manner described have become known in the art as "anti-whirl" bits. Both of the foregoing approaches have also employed the technique of removing rather than adding cutting elements to modify the magnitude and direction of forces in the radial plane perpendicular to the axis of the bit.
While an improvement over the prior art, bits manufactured according to either of the above methodologies still experience operational problems, including but not limited to deviational tendencies, to the point where many of their advantages have yet to be fully exploited, and the cause for such problems has apparently not been ascertained. In addition, the existence of large and directed side forces in anti-whirl bits has been observed by the inventors herein to cause cocking or tilting of the bits in the borehole, and also to cause excessive and premature wear on the portion of the bit gage in contact with the borehole wall, especially at the lowermost portion of the gage.